net: core: devlink: use right genl user_ptr when handling port param get/set

Fix incorrect user_ptr dereferencing when handling port param get/set:

    idx [0] stores the 'struct devlink' pointer;
    idx [1] stores the 'struct devlink_port' pointer;

Fixes: 637989b5

 ("devlink: Always use user_ptr[0] for devlink and simplify post_doit")
CC: Parav Pandit <parav@mellanox.com>
Signed-off-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: Vadym Kochan <vadym.kochan@plvision.eu>
Link: https://lore.kernel.org/r/20210119085333.16833-1-vadym.kochan@plvision.eu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>